The magnesium fluorogermanate phosphor having the formula of 3.5MgO.0.5MgF2.GeO2:Mn4+ is known as a deep red light-emitting phosphor as is described in “Phosphor Handbook” (edited by Phosphor Study Society, published by Ohm Corporation, Dec. 25, 1987, page 19).
JP 2008-202044 A describes a deep red light-emitting magnesium fluorogermanate phosphor having the following formula:(k−x)MgO.xAF2.GeO2:yMn4+in which k is a real number of 2.8 to 5; x is a real number of 0.1 to 0.7; y is a real number of 0.005 to 0.015; and A is Ca, Sr, Ba, Zn or a mixture of these elements.
This patent publication describes a method for producing the above-mentioned phosphor which comprises steps of preparing a homogeneous powdery mixture comprising magnesium oxide, a metal fluoride, germanium oxide and a manganese precursor at a molar ratio of (k−x):x:1:y; calcining the powdery mixture by heating it at 1,000 to 1,200° C. for 4 to 9 hours, and washing and filtering the calcined mixture. It is noted, however, that the publication is silent with respect to details of the magnesium oxide which is employed as the magnesium source. It is also noted that the publication indicates the use of the above-mentioned phosphor as a red light-emitting source for an LED or a fluorescent lamp in which a ultraviolet light source or a blue light-emitting source is utilized.
JP 2011-6501 A describes that a magnesium fluoro-germanate phosphor having the formula of 3.5MgO.0.5MgF2.GeO2:Mn4+ can be modified to vary the ratio of its constitutional elements or to partly replace the constitutional elements with other elements, to give a deep red light-emitting phosphor showing increased efficiency of emission produced when it is excited with a light of the near ultraviolet region or visible region, that is a light in the wavelength region of 350 to 500 nm.
This publication further describes a deep red light-emitting phosphor having a modified ratio of the constitutional elements which is represented by the following formula:xMgO.yMgF2.GeO2:zMn4+in which x, y and z satisfy the conditions of 1.5<x≦4, 0.5<y≦2, 0<z0.1, and y<x.
The above-mentioned publication further describes a deep red light-emitting magnesium fluorogermanate phosphor having been modified by replacing portions of the constitutional elements with other elements which is represented by the following formula:(x−a)MgO.aMe1O.yMgF2.bMe2Hal2.(1−c)GeO2.cMtO2:zMn4+in which x, y, z, a, b and c satisfy the conditions of 1.5<x≦4, 0<y≦2, 0<z≦0.1, 0<a≦1.5, 0<b≦2, o<c<0.5; each of Me1 and Me2 is at least one of Ca, Sr, Ba and Zn, Hal is at least one of F and Cl; and Mt is at least one of Ti, Sn and Zr.
It is noted, however, that this publication does not describe a general description on processes for preparing the above-mentioned phosphor. In the working examples, the phosphor is prepared by employing magnesium carbonate as the magnesium oxide source. The publication further describes the use of the phosphor as the red light-emitting source for a light-emitting diode(LED).